Multi-strand substrate for ball-grid array assemblies and method

ABSTRACT

A multi-strand printed circuit board substrate for ball-grid array (BGA) assemblies includes a printed wiring board ( 11 ) having a plurality of BGA substrates ( 12 ) arranged in N rows ( 14 ) and M columns ( 16 ) to form an N by M array. N and M are greater than or equal to 2 and the size of the N by M array is selected such that each of the plurality of BGA substrates ( 12 ) maintains a planarity variation less than approximately 0.15 mm (approximately 6 mils). The printed wiring board ( 11 ) has a thickness ( 26 ) sufficient to minimize planarity variation and to allow a manufacturer to use automated assembly equipment without having to use support pallets or trays.

BACKGROUND OF THE INVENTION

This invention relates, in general, to semiconductor packages, and moreparticularly, to ball-grid array semiconductor packaging.

Ball-grid array (BGA) semiconductor packages are well known in theelectronics industry. BGA packages provide denser surface mountinterconnects than quad flat pack (QFP) packages. Industry consensus isthat BGA packages are more cost effective than QFP packages forinput/output (I/O) requirements greater than 250. However, there is agreat demand for cost effective BGA solutions down to 100 I/O.

During the assembly of a BGA package, an organic resin printed wiringboard substrate having a thickness on the order of 0.35 millimeters (mm)is placed on a metal pallet or support device. The metal pallet providessupport for the printed wiring board during the majority of assemblysteps. The printed wiring board comprises a single BGA substrate or asingle row or strand of a number of BGA substrates. The largestavailable single strand printed wiring board is a 1×6 printed wiringboard with a maximum total length of about 200 mm. Next, a semiconductordie having a multitude of bonding pads is attached to a die pad locatedon the top side of the BGA substrate. Wire bonds are then attached tothe bonding pads and to bond posts on the top side of the BGA substrate.Next, the semiconductor die and the wire bonds are encapsulated with anorganic material. After encapsulation, the encapsulation material iscured at an elevated temperature. Conductive solder balls are thenattached to contact pads, which are on the lower side of the BGAsubstrate and electrically coupled through conductive traces to the bondposts, using a solder reflow process. Each BGA package is then marked.When a single strand of multiple BGA packages is used, a singulationprocess such as a punch press is used to separate the multiple BGApackages into single units.

The above assembly process has several disadvantages. Because the aboveprocess requires a metal pallet to support the thin BGA substratesduring the majority of assembly steps, the process is not conducive tolarge scale automated assembly. As a result, manufacturers must purchaseadditional equipment to assemble BGA packages. This requires capitalinvestment in equipment and additional factory space. Also, because onlysingle substrates or a single strand of a several substrates is used, itis difficult for manufacturers to produce a large volume of BGA packagesefficiently. In addition, the above process requires significant laborinputs to load and unload the metal pallets or support devices at thevarious process steps. This negatively impacts manufacturing cycle timeand quality. Furthermore, the pallets are expensive because they requireprecise tolerances for use with automated equipment and they require amanufacturer to carry a large inventory to support work-in-process (WIP)throughout a manufacturing line.

Industry standards require that after assembly, each BGA substrate mustmaintain a planarity variation of less than approximately 0.15 mm(approximately 6 mils) as measured at three points across a substrate.In other words, each BGA substrate must not be excessively warped ornon-planar. Because of this strict standard and a concern over warpage,printed wiring board suppliers and BGA semiconductor manufacturers havenot been motivated to expand beyond the existing 1×6 single strandprinted wiring board.

With the rapid increase in demand for BGA packages, it is readilyapparent that a need exists for cost effective printed wiring boardsubstrates that are conducive to large scale automated assembly, thatsupport existing automated assembly equipment, and that do not warpduring the assembly process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a top view of an embodiment of a printed circuitboard substrate for BGA assemblies according to the present invention;and

FIG. 2 illustrates an enlarged cross-sectional side view of one BGAassembly according to FIG. 1.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention can be better understood with reference to FIGS. 1and 2. FIG. 1 illustrates a top view of multi-strand substrate, printedcircuit board, or wiring board (PCB) 11. PCB 11 typically comprises anorganic epoxy-glass resin based material, such as bismaleimide-triazin(BT) resin, FR-4 board, or the like. PCB 11 includes BGA substrates orpatterned package substrates 12 arranged in N rows 14 and M columns 16to form an N by M pattern or array. Each of BGA substrates 12 includes adie attach or bonding pad 13, which typically comprises copper or goldplated copper. Die attach pad 13 is a solid metallization area or apatterned metallization area shaped like a cross, “Union Jack”, or otherspecialized geometry. To avoid overcrowding the drawing, conductivetraces are not shown (conductive traces are shown in FIG. 2). PCB 11 isformed using well known printed circuit board manufacturing techniques.

To support efficient large scale automated assembly, N and M arepreferably at least greater than or equal to 2. Depending on the finaldimensions of BGA substrates 12, N and M are selected such that afterall assembly steps are completed, each of BGA substrates 12 exhibit aplanarity variation of less than approximately 0.15 mm across each ofBGA substrates 12. In other words, during assembly, each of BGAsubstrates 12 does not warp to a non-planar condition in excess ofapproximately 0.15 mm. Also, PCB 11 has a thickness 26 (shown in FIG. 2)sufficient to minimize warpage or non-planarity. As stated below,thickness 26 preferably is on an order of at least 0.5 mm. According tostandard industry practice, warpage within a given unit is determined bymeasuring a maximum difference between a seating plane (formed by thethree conductive solder balls (see FIG. 2) having the greatest amount ofstandoff from the BGA substrate) and the conductive solder ball with theleast amount of stand-off from the substrate. The warpage measurement istaken after PCB 11 is separated into individual BGA units.

Preferably, PCB 11 further includes a plurality of stress-relief slotsor slots 19 at various locations on PCB 11. Preferably, slots 19 extendthrough PCB 11. Slots 19 are all the same size or of different sizes.Slots 19 further minimize warpage of each of BGA substrates 12. Also,PCB 11 preferably includes alignment holes 21 along one side or bothsides of PCB 11. Alignment holes 21 extend from the top surface to thelower surface of PCB 11. Alignment holes 21 are placed according to therequirements of die attaching and wire bonding equipment to supportautomated assembly. Additionally, PCB 11 preferably includes holes 22around the perimeter of PCB 11 and hole 23 along one side of PCB 11.Holes 22 provide for an automatic orientation feature so that amanufacturer does not insert PCB 11 into assembly equipment backwards orreversed. Hole 23 provides for an orientation feature to allow amanufacturer to robotically place PCB 11 in a jig apparatus.

In a preferred embodiment for a 27 mm by 27 mm BGA device, N is equal to2 and M is equal to 6 with PCB 11 having a length 17 on an order of 187mm and a width 18 on an order of 63 mm. The above specifications alsoare preferred for a 23 mm by 23 mm and a 25 mm by 25 mm BGA device. In apreferred embodiment for a 9 mm by 9 mm BGA device, N is equal to 4 andM is equal to 12 with length 17 on an order of 200 mm and width 18 on anorder of 63 mm. In a preferred embodiment for a 10.4 mm by 10.4 mm BGAdevice, N is equal to 4 and M is equal to 12 with length 17 on an orderof 212 mm and width 18 on an order of 63 mm. In a preferred embodimentfor a 15 mm by 15 mm BGA device, N is equal to 3 and M is equal to 9with length 17 on an order of 187 mm and width 18 on an order of 63 mm.In a preferred embodiment for a 14 mm by 22 mm BGA device, N is equal to2 and M is equal to 9 with length 17 on an order of 187 mm and width 18on an order of 63 mm. Optionally, for a 14 mm by 22 mm BGA device, N isequal to 3 and M is equal 6 with length 17 and width 18 the same asabove. In a preferred embodiment for a 35 mm by 35 mm BGA device, N isequal to 1 and M is equal to 4 with length 17 on an order of 187 mm andwidth 18 on an order of 63 mm. The above dimensions are preferred totake advantage of standard automatic assembly equipment requirements.This allows a manufacturer to use existing tooling and equipment. Theabove dimensions are easily modified to meet the requirements ofdifferent types of automated assembly equipment.

FIG. 2 illustrates an enlarged cross-sectional view of one BGAstructure, assembly, or package 22 after assembly but before singulationor separation into individual packages. BGA structure 22 comprises oneof BGA substrates 12 within PCB 11. PCB 11 with BGA substrates 12preferably has a thickness 26 such that PCB 11 can undergomagazine-to-magazine automated assembly processes without using a metalsupport pallet. Currently available single BGA substrate PCB's andsingle strand BGA substrates PCB's have thicknesses on the order of 0.35mm, which is too flimsy for reliable automated assembly unless palletsor carriers are used. Thickness 26 also is selected to minimizeplanarity variation of each of BGA substrates 12. Preferably, thickness26 is greater than approximately 0.5 mm. Preferably, thickness 26 is ina range from approximately 0.5 mm to approximately 0.8 mm.

BGA structure 22 further includes a semiconductor die 24 attached dieattach pad 13 on an upper surface of each of BGA substrates 12.Semiconductor die 24 has a plurality of bonding or bond pads 28. Each ofBGA substrates 12 has a conductive connective structure comprising bondposts 31, upper conductive traces 32, vias 33, lower conductive traces36 and contact pads 38. Conductive solder balls 41 are attached tocontact pads 38. Conductive wires or wire bonds 43 electrically couplebond pads 28 to bond posts 31. Alternatively, semiconductor die 24 ismounted in a “flip-chip” embodiment with bond pads 28 directly connectedto bond posts directly below bond pads 28, eliminating conductive wires43 and die attach pad 13. An encapsulating layer or encapsulant 46covers semiconductor die 24 and wire bonds 43 to provide protection ofactive circuit elements from physical damage and/or corrosion.

A typical BGA assembly process incorporating PCB 11 having BGAsubstrates 12 to form BGA structure 22 is described as follows. FirstPCB 11 is provided having the desired N by M pattern. PCB 11 is loadedonto an automated die attach machine such as an ESEC 2006. This type ofdie attach machine is an industry standard machine that manufacturersuse to attach semiconductor die to other types of semiconductor packagessuch as plastic dual-in-line (PDIP), small outline integrated circuit(SOIC), and QFP packages. The die attach machine automatically attachesone semiconductor die 24 to one of die attach pads 13 on PCB 11.Preferably, semiconductor die 24 is attached to one of die attach pads13 using a die attach epoxy.

After die attach, PCB 11 is then cleaned using an automated cleaningsystem such as an ULVAC cleaning system available from the ULVAC Corp.Next, PCB 11 is placed on an automated wire bonder such as a ShinkawaUTC-100 where wire bonds 43 are attached to bond pads 22 and bond posts31. In conventional BGA processing, wire bonding is done using a similarwirebonder configured for semi-automatic operation.

Next, encapsulant 46 is applied to cover semiconductor die 24 and wirebonds 43. Encapsulant 46 comprises an organic material and is appliedusing an over-mold process or a glob-top process. For an over-moldprocess, automolds from Towa, Fico, or similar suppliers are used. Whenan over-molding process is used, encapsulant 46 preferably comprises anorganic mold compound. When a glob-top process is used, encapsulant 46preferably comprises an anhydride epoxy organic compound. Preferably,the material selected for encapsulant 46 has a thermal coefficient ofexpansion (TCE) close (within a few points or parts per million) to theTCE's of the material of PCB 11 and semiconductor die 24. This furtherhelps to minimize warpage of BGA substrates 12 during the remainder ofthe assembly process. Such encapsulents are available from severalsuppliers including The Dexter Corp., of Industry, Calif., Ciba-GiegyCorp., Hitachi Corp., Sumitomo Corp., and Nitto-Denko Corp.

Next, encapsulant 46 is cured preferably using a belt furnace, verticaloven, or batch oven at a temperature that is function of the type ofmaterial used for encapsulant 46. For the curing process, a tray or someother form of protection preferably is used to protect contact pads 38from foreign matter contamination.

After encapsulation, conductive solder balls 41 are attached to contactpads 38 using a room temperature attach process. Next, an automatedsolder reflow process is used to reflow conductive solder balls 41.Automated reflow/equipment such as a belt furnace. After reflow, PCB 11is again cleaned using automated cleaning equipment with an aqueous orterpene media to remove any corrosive flux residues from the conductivesolder ball attachment process. Each BGA structure 22 is then marked onan automated marking machine such as an automated laser marker.Optionally , marking occurs immediately following encapsulation. Finallyeach of BGA substrates 12 is divided into individual packages. To dividethe packages, a punch-press process is used. Optionally, a routing,dicing, or snapping separation process is used.

By now it should appreciated that there has been provided a multi-strandPCB containing an N by M array of BGA substrates for manufacturing BGAtype semiconductor packages. N and M are selected and the thickness ofthe PCB is such so as to enable enhanced manufacturing efficiency. Theenhanced manufacturing efficiency comes from the ability to manufacturemore BGA packages from one multi-strand PCB and from the ability to usestandard automated assembly equipment. Manufacturing efficiency isenhanced while still providing BGA substrate planarity variation lessthan approximately 0.15 mm. Because the multi-strand PCB according tothe present invention enables the use of standard automated assemblyequipment, a BGA manufacturer is able to use the same types of equipmentto manufacture different types of packages thus reducing capitalexpenditures and needed factory floor space. Also, labor costs arereduced and quality is increased because of the reduced handlingrequirements.

What is claimed is:
 1. A method for assembling ball-grid array (BGA) packages, comprising the steps of: providing a plurality of BGA substrates arranged in an N by M array within a printed circuit board having a thickness, wherein N and M are greater than or equal to 2, each of the plurality of BGA substrates having a plurality of bond posts on one side and a plurality of contact pads on an opposite side; attaching a semiconductor die to each of the plurality of BGA substrates, the semiconductor die having a plurality of bond pads; encapsulating the semiconductor die with an encapsulant; curing the encapsulant; attaching conductive solder balls to each of the plurality of contact pads; and dividing the N by M array into separate BGA packages, and wherein each of the separate BGA packages is substantially planar.
 2. The method of claim 1 wherein the step of encapsulating the semiconductor die includes encapsulating with an encapsulant having a thermal coefficient of expansion close to that of the semiconductor die and the printed circuit board, and wherein size of the N by M array and the thickness are such that each of the plurality of BGA substrates maintains a planarity variation less than approximately 0.15 mm after assembly.
 3. The method of claim 1 wherein the step of providing the plurality of BGA substrates arranged in the N by M array within the printed circuit board includes providing the plurality of BGA substrates within a printed circuit board having a plurality of stress-relief slots at various locations within the printed circuit board.
 4. The method of claim 1 wherein the step of providing the plurality of BGA substrates arranged in the N by M array within the printed circuit board includes providing the plurality of BGA substrates within a printed circuit board having a thickness in a range from approximately 0.5 mm to approximately 0.8 mm.
 5. The method of claim 1 wherein the step of providing the plurality of BGA substrates arranged in the N by M array within the printed circuit board includes providing the plurality of BGA substrates within a printed circuit board having a width on an order of 63 mm.
 6. The method of claim 1 wherein the step of providing the plurality of BGA substrates arranged in the N by M array within the printed circuit board includes providing the plurality of BGA substrates within a printed circuit board having a length in a range from approximately 187 mm to 212 mm.
 7. The method of claim 1 further comprising the steps of: bonding conductive wires to the plurality of bond pads and the plurality of bond posts after the step of attaching the semiconductor die; and marking the BGA packages after the step of encapsulating the semiconductor die.
 8. The method of claim 1 wherein the step of providing the plurality of BGA substrates arranged in the N by M array within the printed circuit board includes providing the plurality of BGA substrates within a printed circuit board comprises of an organic resin. 